There are a broad variety of products where abrasive or erosive wear significantly limits the lifetime of the product. For example, the nozzles used for sand blasting have a continual stream of sand or other abrasive grit entrained in a fluid stream that passes through the nozzle at high velocity. Engagement of the grit particles on the nozzle surface may rapidly erode the hardest materials used to make such nozzles. Another nozzle subject to such wear is used in bits for drilling oil wells or the like. A drilling mud is pumped through such nozzles at reasonably high velocity and the nozzles are eroded both in the orifice through which the mud passes and on the exposed face external to the bit.
A variety of other surfaces on bits are also subject to wear from abrasive rock particles as a bit is rotated in a well. In some production wells high velocity gas with entrained particles may be produced and erosion of production equipment may be encountered. This may include hardware down hole or at the ground surface. Valves or chokes are particularly susceptible to such wear because flow velocities in the valve can be quite high, and complex geometries can cause impingement of fluid streams on surfaces that are thereby subjected to extreme wear conditions.
Adjustable chokes play an important role in controlling the rate of production of oil and gas. Such an adjustable choke is placed in the flow line at a wellhead to provide an adjustable orifice or opening through which fluid flows from the well at a rate limited by the size of the opening. Some, but not all, adjustable chokes are intended to operate as a high pressure valve that can close completely. If desired, a separate valve can be placed in the same fluid flow system upstream from the adjustable choke to shut off flow to the adjustable choke.
Some internal parts within an adjustable choke are often exposed to extremely erosive conditions. In particular, certain internal parts which are often called the "trim" cooperate to define the orifice that limits the fluid flow rate through the choke. The flow velocity of the fluid as it passes through that orifice can be quite large; sonic velocities can occur. Such a high fluid flow velocity presents a highly erosive condition. Another possible erosive condition involves particles entrained in the fluid. Either or both of these erosive conditions may be present.
U.S. Pat. No. 4,337,788, assigned to the assignee of this invention, discloses and claims an earlier invention concerning a high pressure valve. One of the advantages of such a high pressure valve arises from the trim structure involved in defining the orifice. This structure includes an elongated plug and a liner sleeve having an elongated bore coaxial with the plug. The plug is continuously movable along its axis between two end positions. At one position, a portion of the plug engages a seat portion of the liner sleeve and closes the valve. At all other positions of the plug, the valve is open, and the size of the orifice increases as the plug moves toward its opposite end or fully open position. Certain features of the plug and the liner sleeve have the effect of localizing erosion of the liner sleeve in regions downstream of the seat portion. This provides the advantage of eliminating leakage when the partly eroded valve is closed. Further, as disclosed in the above-identified patent, such a plug and liner sleeve are preferably arranged within an assembly that is easily removed and replaced. This has been preferred because the plug and liner sleeve typically have worn out much sooner than other parts of the valve.
The above-identified patent discloses the use of cemented tungsten carbide as the material for both the plug and for the liner sleeve. Tungsten carbide is harder than sand that may be entrained in the fluid flowing through the valve, and accordingly would be expected to resist wear from such sand as well as any material that is softer than the sand. Notwithstanding the foregoing, such cemented tungsten carbide plugs and liner sleeves have had a short useful life in some environments. For example, such a plug and liner sleeve wore out in approximately four to five days of use in a gas well producing sandy gas. There is accordingly a substantial need for a high pressure valve having a longer useful life.
Another type of valve used in very high pressure systems and subject to extremely erosive conditions because of high velocity flow is a so-called plug and cage valve. In this type of valve fluid is introduced in the valve casing outside of a hollow cylindrical cage. Various sizes and shapes of orifices lead through the wall of the cage to its interior. A tight fitting plug can move axially in the cage to alternately occult or clear such orifices to block or permit flow through the cage. The plug position in the cage controls the flow through the valve. The edge of the plug in such a valve may be eroded and change the flow through the valve or prevent complete closure of the valve. The cage may be subject to appreciable erosion on the inside around the orifices, apparently due to eddies in the liquid filling the valve.
A broad variety of other surfaces are also subject to erosive wear by abrasive materials or rapidly flowing fluids, some of which may contain abrasive particles. It is highly desirable to protect such surfaces from erosion to prolong the life of the equipment.
Erosion resistance is provided on many such surfaces by constructing the parts of ceramic, cemented tungsten carbide or similar hard material. Generally speaking the harder the material the better it resists erosion. It would be desirable to form erosion and wear resistant articles out of diamond since it is the hardest known material. Cost is not the only reason valve parts and the like are not made of diamond. One cannot make large diamond parts or parts of very complex geometry, regardless of cost.
Techniques have been developed for making polycrystalline diamond products by subjecting a mass containing diamond crystals to high temperatures at sufficiently high pressure that diamond is thermodynamically stable. In effect, the diamond crystals are "welded" together to form a strong polycrystalline mass. For example, U.S. Pat. No. 3,141,746 by DeLai describes formation of polycrystalline diamond (often referred to as PCD). Techniques have also been developed for forming polycrystalline diamond layers on substrates such as cemented tungsten carbide. For example, U.S. Pat. Nos. 3,745,623 by Wentorf, 3,831,428 by Wentorf and 3,850,053 by Bovenkerk illustrate formation of flat layers on a carbide substrate. U.S. Pat. No. 4,109,737 by Bovenkerk illustrates formation of non-flat layers.
The size of parts that can be made with polycrystalline diamond is limited by the need for maintaining extremely high pressures when forming PCD. This has necessarily limited the size of presses capable of reaching such high pressures. Pieces about thirty millimeters across are among the largest made to date. Further, in the presses employed in the techniques described in the above-mentioned patents, isostatic pressure is not obtained and complex shapes do not appear to be feasible. Isostatic pressure is more nearly obtained in presses as described in U.S. Pat. Nos. 2,918,699; 3,159,876 and 3,182,353, for example.
Thus, it is desirable to form valve parts and other surfaces subject to wear and erosion out of diamond but techniques for doing so are not readily available.